1. Field of the Invention
The subject invention relates to structural composites. More particularly, the invention relates to structural composites prepared from maleimide resins cured with organophosphine or organophosphonium catalysts such as triphenylphosphine, or complexes thereof.
2. Description of the Related Art
Advanced composites and adhesives are becoming increasingly important in fabricating structures, especially in the aerospace, transportation and electrical industries. Many of the modern construction techniques utilized in these industries bear little resemblance to those practiced even as recently as a decade ago. Whole aircraft wing structures, for example, may be fabricated by laminating together a variety of woven fibrous materials impregnated with resin. The fibers useful for these purposes run the gamut from simple cotton or dacron to exotic and expensive fibers such as boron, carbon, and graphite.
The resins and adhesives necessary to form the composites and to bond them to each other must possess a variety of sometimes conflicting physical properties. They must form a strong resin-to-fiber bond while having great tensile strength, modulus, and thermal stability. Modern resins are far superior in this respect to the resins previously used in composite construction.
Among the resins commonly used for advanced structural composites are the bis-maleimide resins. These resins may be produced by the reaction of maleic anhydride with a suitable diamine. Suitable diamines, for example, are common aliphatic and aromatic diamines. The production of suitable bis-maleimide monomers is described, for example, in U.S. Pat. Nos. 3,839,358; 3,018,290; 3,018,292; and 3,627,780. The bis-maleimides produced correspond to the formula: ##STR1## wwherein R is hydrogen, lower alkyl or substituted lower alkyl, and where A is the organic residue derived from the diamine. These bis-maleimides may be the sole monomer in the resin system and may be polymerized through the application of heat alone or in conjunction with suitable catalysts.
Unfortunately, polymerization of bis-maleimides alone results in polymers which, a1though strong, tend to be brittle. Thus attempts have been made to copolymerize bis-maleimides with other monomers. In U.S. Pat. No. 3,562,223, for example, bis-maleimides are co-polymerized with organic diamines in a bis-maleimide to diamine ratio of from 1.2:1 to 50:1. ln U.S. Pat. Nos. 4,518,754 and 4,518,755 the co-polymerization of bis-maleimides and olefinically unsaturated monomers is disclosed. Co-polymerization of bis-maleimides with alkenylphenols or alkenylphenol ethers is disclosed in U.S. Pat. No. 4,100,140, while co-polymerization with N-vinylpyrrolidin-2-one and acrylamide is disclosed in U.S. Pat. No. 4,413,107.
The bis-maleimides may also be in the form of maleimide terminated prepolymers. When such prepolymers are utilized, the prepolymer chain length and type may be selected to provide optimal performance. Long non-polar prepolymers tend to possess high elongation while shorter, polar prepolymers tend to have greater modulus. A wide variety of such bis-maleimide prepolymers are commercially available.
While most of these resin systems may be polymerized through the application of heat alone, generally catalysts are necessary to achieve optimum performance. Suitable catalysts are the various secondary, tertiary, and quaterary amines as disclosed in U.S. Pat. No. 4,100,140. Examples of these are the various dialkyl and trialkyl amines such as di- and triethylamine. Various peroxy ketals useful in the polymerization of polyimide systems are disclosed in U.S. Pat. No. 4,338,430, and alkali metal salt catalysts, particularly the salts of mono- and dicarboxylic acids are disclosed in U.S. Pat. No. 4,418,181. The use of zinc octoate, dimethylbenzylamine, and 2-ethyl-4-methylimidazole, alone and in combination, is disclosed in U.S. Pat. No. 4,410,601.
However, despite the wide variety of catalysts proposed in the prior art, further improvement is still desirable. The reason why catalysis of the various bis-maleimide resin systems is particularly difficult lies in the manner in which these resins are customarily used. For example, the polyimide systems contemplated by the subject invention are often prepared in the form of films coated or cast on release paper for later use. In these applications, the resin may be deposited from a solution in an organic solvent, as a dispersion, or preferably, as a melt.
The films thus prepared are generally used for the preparation of fiber reinforced prepregs. The bis-maleimide resin film is fed, along with the yarn, tape, or cloth desired to be impregnated, through a system of heated pressure rollers. A single film may be applied to the top or bottom of the yarn, tape or cloth substrate, or two films may be applied, one of the top and one on the bottom. These processes of prepreg formation are well known to those skilled in the art of advanced structural composites.
Instead of preparing a film, an alternative method is to deposit the resin system directly onto the yarn, tape, or cloth. This may be accomplished by the use of solution techniques where the resin is dissolved in a solvent, dispersion techniques where the resin is dispersed in a suitable continuous phase; or directly from the neat resin in the melt. These techniques are often performed at moderately elevated temperatures.
Because the prepregs prepared by these processes contain partially cured systems containing latent or low room temperature activity cure catalysts, they must generally be stored at low temperatures prior to their use in manufacturing composites. Such prepregs often lose their tack and drape characteristics after only a short period of time. Even when carefully stored, their storage life is generally limited. Shelf lives of only several days at room temperature are common. Refrigeration may be useful in extending this time somewhat, but prepregs having extended room temperature shelf life are needed.
If the catalyst utilized if the resin is too active at low temperatures, resin in the prepreg may polymerize during the preparation of the resin or during the prepreg impregnation processes, producing resins and prepregs which are too highly advanced and therefore virtually infusible. These prepreg would not be suitable for their intended use in economically fabricating structural composites. However, if the catalyst is too inactive during cure, excessive curing temperatures may be required in order to prevent long gel times when the prepregs are later assembled and cured into composites.
Composites prepared by laminating resin impregnated yarn, tape, or fabric prepreg are often subjected to a partial cure while in the mold or form used to shape the composite structure followed by a second cure. often at an elevated temperature. The green strength of the partially cured composite is of importance, as composites with higher green strength are handled more easily and are superior with respect to various machining operations often performed at this stage.